Hydrocarbon conversion process, naphtha to aromatics and town gas



Feb 27. 1968 D. B. CARSON HYDROCARBON CONVERSION PROCESS, NAPHTHA TO AROMATICS AND TOWN GAS Filed Aug. 19, 196e United States Patent CONVERSION PRoCESS, AnoMArrCs AND TowN Don B. Carson, Mount Prospect, Ill., assignor to Universai Oil Products Company, Des Plaines, Ill., a corporation of Delaware Filed Aug. 19, 1966, Ser. No. 573,560 9 Claims. (Cl. 26d-672) HYDRCAREON NAPHTHA TO GAS ABSTRACT OF THE DISCLSURE This invention relates to a hydrocarbon conversion process wherein an aromatic hydrocarbon and town gas are produced as co-products. It particularly relates to a combination method embodying the processes of reforming and dealkylation to produce benzene-type hydrocarbons and town gas. It specifically relates to a unified combination process for variably adjusting the calorific value of town gas while simultaneously producing benzene in high concentration from a petroleum-derived hydrocarbon feedstock.

The production of utility gas, or town gas, for use in household or factory consumption, is well known to those skilled in the art. Town gas can be produced by numerous processes from various raw materials depending largely upon the available raw materials in the particular locality. In similar manner, the production of aromatic hydrocarbons is also well known to those skilled in the art. Aromatic hydrocarbons such as benzene are in great demand by industry in general for a variety of useful purposes in the chemical industry.

As will be more fully developed hereinbelow, the production of town gas is difcult to control since the various constituents which make up such a gas each have different heats of combustion or calorifc Vahle. For obvious reasons, the heating value of the town gas must be fairly carefully controlled to t the requirements of the equipment in which it is to be burned. If gas of low B.t.u content is produced it is usually enriched by another hydrocarbon of higher calorifc value, such as propane, and, conversely, if gas of excessively high B.t.u. content is produced it is usually diluted with an inert gas, such as a stack gas.

Due to consumer demand, the public utilities have in many cases been unable to supply an adequate amount of gas for periods of peak demand. Therefore, there is a demand for a more ecnomical and flexible process which would allow the production of town gas under variable conditions.

The production of an aromatic hydrocarbon such as benzene is susceptible to fluctuating demands of the market place. In recent times, however, the demand for benzene has far outstripped the capacity of existing units to produce it. The prior art processes which produced aromatic hydrocarbons, such as the family of alkyl benzenes, were not sufficient to supply all of the benzene necessary to meet the demands of industry. Accordingly, the toluenes and mixed Xylenes were subjected to additional processing such as dealkylatiou in order to increase the supply of benzene.

Accordingly, it is an object of this invention to provide a process for the conversion of hydrocarbons wherein aromatic hydrocarbons and town gas are produced.

It is another object of this invention to provide a process for. the simultaneous production of benzene and town gas in a more economical and facile manner than has heretofore been possible.

It is still another object of this invention to provide a method which embodies a combination process of catalytic reforming and dealkylation whereby benzene is produced and town gas is adjusted to predetermined calorifi-c value.

These and other objects of the invention will be obvious to those skilled in the art from the discussion presented hereinbelow with reference to the appended drawing which is a schematic representation of an illustrative embodiment of the invention.

According to the present invention these and other objects are attained by a process which comprises converting a hydrocarbon fraction under conditions sufficient to produce a hydrogen-rich stream and a carbon dioxiderich stream; reforming a hydrocarbon stream containing naphthenic and paraf'lnic hydrocarbons under conditions including the presence of at least a portion of said hydrogen-rich stream sufficient to produce a reformate containing alkylaromatic hydrocarbons; dealkylating the alkylaromatic hydrocarbons under conditions sufficient to produce an effluent containing aromatic hydrocarbons and a normally gaseous stream of relatively'low purity hydrogen gas; separating at least a portion of the relatively low purity hydrogen gas so produced into a relatively high purity hydrogen stream and a light parailinic hydrocarbon stream; recycling said purified hydrogen stream to the reforming step as part of said conditions; admixing at least a portion of the carbon dioxide-rich stream with said light hydrocarbon stream in an amount sufficient to produce town gas having a predetermined caloric value; recovering aromatic hydrocarbons in high concentration from said effluent, and recovering me town gas so produced.

Another embodiment of this invention provides for passing the entire effluent from the reforming step to the dealkylation step without intervening separation.

Still another embodiment of this invention includes separating the relatively low purity hydrogen gas by cyrogenic means into a high purity hydrogen stream and a town gas having a calorific value of from 300 to 1000 B.t.u.s per standard cubic foot (B.t.u./s.c.f.).

A particular embodiment of this invention provides a combination method for producing town gas and benzene which comprises the steps of: (a) steam cracking a naphtha `fraction to produce a normally gaseous stream comprising carbon dioxide and a gaseous stream comprising at least by volume hydrogen; (b) catalytically reforming a naphtha fraction boiling between F. and 450 F. and containing primarily naphthenic and paraffinic hydrocarbons in the presence of at least a portion of said gaseous hydrogen stream under conditions including a temperature from 500 F. to 1050 F., a pressure from 50 p.s.i.g. to 1200 p.s.i.g., and a weight hourly space velocity from 0.2 to 40, preferably from 0.5 to 20, sufficient to produce a reformate containing alkylbenzene hydrocarbons and normally gaseous parafiinic hydrocarbons; (c) subjecting said reformate to catalytic dealkylating conditions including the presence of hydrogen, a temperature from l000 F. to 1500 F., a pressure from 100 p.s,i.g. to 1000 p.s.i.g., a weight hourly space velocity from 0.1 to 20.0, and a hydrogen-to-hydr-ocarbon mol ratio from 3:1 to 10:1, whereby alkylbenzene hydrocarbons are converted to benzene; (d) separating the efiiuent from the hydrodealkylation step into a liquid hydrocarbon stream comprising benzene and a gaseous fraction comprising from 40% to 70% by volume hydrogen; (e) separating at least a portion of said gaseous fraction Iby cryogenic means into a hydrogen recycle stream comprising at least 80% by volume hydrogen and a light paraffinic hydrocarbon stream; (f) passing said recycle stream to the catalytic reforming step (b) as part of said conditions; and (g) admixing light paraffnic hydrocarbons from step e with at least a portion of the carbon dioxide stream from step a to produce a town gas having a predetermined calorific value.

Thus, it can be seen that the present invention embodies the interrelated and interdependent processing steps of hydrogen production, catalytic reforming, dealkylation, and hydrogen purification to produce a unified process whereby town gas having a predetermined rcalorific value is produced while simultaneously producing aromatic hydrocarbons such as benzene in high concentration.

The production of hydrogen, per se, is well known Ito those skilled in the art. For example, U.S. Patent No. 2,750,261, Ipatieff et al., teaches a process for the production of hydrogen by the interaction of an aliphatic hydrocarbon and steam at elevated temperatures in the presence of catalytic material. As can be seen from the stoichiometry presented by the patentees, hydrogen and carbon dioxide are the sole products from the steam cracking of hydrocarbons. Even though the early prior art processes were limited to the steam cracking of methane, recent advances have provided techniques for the steam cracking of liquid hydrocarbons such as a naphtha fraction derived from petroleum. Typically, hydrogen is produced in a single stage process at a temperature below 700 C. by using a molar ratio of steam to hydrogen of about in the presence of a catalytic component comprising nickel. Those skilled in the art are familiar with the production of hydrogen and carbon dioxide in high concentration and, therefore, production of these components, per se, form no part of this invention.

For the production of hydrogen and carbon dioxide in accordance with this invention, various feedstocks may be satisfactorily used. A feedstock containing aliphatic and aromatic hydrocarbons from methane to higher molecular weight hydrocarbons, including acyclic and alicyclic, parafinic and olefinic organic compounds, such as those containing up to about 40 carbon atoms per molecule or molecular weights as high as about 560, may be used with satisfactory results. The feedstock may be a single hydrocarbon, such as ethylene, ethane, propane, propylene, hexene, hexane, n-heptane, etc., and mixtures thereof including vairous petroleum derived fractions such as light naphtha (e.g. boiling range from about 100 F. to 250 F.), heavy naphtha (eg. boiling range of about 200 F. to 400 F.), gas oil (eg. boiling range of about 400 F. to 700 F.), as well as mineral oils, crude petroleum, including topped and residual oils, and renery and coke oven gases. It is distinctly preferred in the practice of this invention that a light naphtha fraction be used for the -production of hydrogen and carbon dioxide.

The operating conditions to effect the production of hydrogen in relatively high purity are well known to those skilled in the art. Generally, it is a catalytic process, as noted previously, and will utilize temperatures in the range of about 600 F. to 1800 F. Space velocities are based on methane equivalents per hour volume of catalyst and typically will range between 50 to about 1000.

The amount of steam required to produce hydrogen is also well known and frequently is expressed in terms of a steam-to-carbon ratio which is the number of steam molecules charged to the reaction zone per atom of carbon charged. A satisfactory range of steam-to-carbon ratios will be from about 1.5 to about 5.0 for the range of charge stocks contemplated hereinabove.

From the reforming of petroleum-derived feedstocks for the production of hydrogen, the first stage produces hydrogen and carbon dioxide as well as large quantities of carbon monoxide. As indicated by the stoichiometry referred to in the lpatiefl` et al. patent, supra, it is conventional to contact carbon monoxide with a catalyst, such as iron oxide, to convert the carbon monoxide to carbon dioxide with an equivalent amount of hydrogen also being produced. The carbon dioxide may then be separated from the hydrogen by well known methods, e.g. absorption with an alkanol amine such as triethanolamine.

Suitable charge stocks for use in the catalytic reforming step to produce alkyl aromatic hydrocarbons, are those which contain both naphthenes and paraflins in relatively high concentration. Suitable stocks include narrow boiling fractions as well as substantially pure materials such as cyclohexane and methylcyclohexane. The preferred class includes primarily straight run gasolines including the light and heavy naphthas mentioned hereinabove with respect to the hydrogen producing step. It is distinctly preferred to use a naphtha fraction boiling between F. and 400 F. as the feedstock to the catalytic reforming step.

It is to be noted that the practice of this invention may achieve excellent results with the same feedstock being used for both the hydrogen producing step and the catalytic reforming step to produce alkyl aromatic hydrocarbons. As will be more fully discussed hereinbelow with reference to the appended drawing, such as feedstock is simply split between the two operations in amounts equivalent to the required hydrogen and carbon dioxide which is necessary to produce the benzene and town gas having a predetermined calorifc Value.

The preferred types of catalyst for use in the catalytic reforming operation are well known to those skilled in the art and typically comprise platinum and alumina. These catalysts may contain substantial amounts of platinum, but for economic and quality reasons the platinum will usually be within the range of from 0.05% to about 5% by weight.

The hydrocarbon reforming operations to produce alkylaromatic hydrocarbons may be carried out in the presence of a catalyst at temperatures from about 500 F. to about 1050 preferably from 600 to 1000 F., a pressure from about 50 to about 1200 p.s.i.g., preferably from about 200 to 600 p.s.i.g., a weight hourly space velocity within the range of about 0.2 to about 40, and in the presence of a hydrogen-containing gas equivalent to a hydrogen-to-hydrocarbon mol ratio of about 0.5 to about l5.

In accordance with this invention, the preferred charge stock for use in the dealkylation step is the entire effluent from the catalytic reforming step without intervening separation. There are significant advantages to passing the entire effluent from the reformer to the dealkylator in that all intervening usual fractionating towers, coolers, and condensers may be dispensed with. Also, since the catalytic reforming step is a hydrogen producer, there are suiiicient quantities of hydrogen present in the effluent from the catalytic reformer to satisfactorily effectuate the hydrodealkylation reaction. lf desired, however, the usual separation equipment may be placed between these two processing units in order to prepare a hydrocarbon fraction rich in alkylaromatic hydrocarbons and boiling generally in the range of 200 F. to 600 F., preferably in the range of 200 F. to 400 F. The latter boiling range material will contain the predominant amount of alkyl benzenes which will become dealkylated in the dealkylation Zone to benzene which in the preferred embodiment of this inyention is the desired production.

The dealkylation reaction may be effectuated either in the presence of a suitable catalyst or, if desired, in the substantial absence of catalyst, e.g. thermally. In either case the operating conditions are well known to those skilled in the art and include a temperature in the range of from 1000 F. to 1800 F., a pressure of between 100 and 1000 p.s.i.g., and in the presence of at least one molecule of hydrogen per atom of alkyl carbon in the alkylated aromatic feed, i.e. a hydrogen-to-hydrocarbon mol ratio within the range of 3:1 to 25:1, preferably 5:1

to :1. For the thermal operation, additionally, a residence time of from 2 to 300 seconds, preferably 10' to 60 seconds, in the reaction zone should be satisfactory. For the catalytic operation, a weight lhourly space velocity of from 0.l to 20, preferably from about 0.5 to 5, will produce satisfactory results. In each case the operation conditions should be sufficient to dealkylate at least a portion of the alkylated aromatic hydrocarbons.

The eiiiuent from the dealkylation zone is Withdrawn from the zone and passed, generally, to a high pressure separator wherein the efiiuent is separated into a hydrogenrich gaseous fraction and a liquid hydrocarbon fraction which contains the desired aromatic hydrocarbon, e.g. benzene. The yhydrogen-rich gas is contaminated with significant quantities of relatively light paraffinic hydrocarbons. Accordingly, for economy of operation and to maximize the catalyst life in the catalytic reforming zone, this hydrogen-rich stream is passed at least in part to a hydrogen purification zone wherein a hydrogen stream of at least 80% by volume hydrogen is obtained for recyclingto the catalytic reforming zone.

The purification or separation of this hydrogen-rich stream is performed by means known to those skilled in the art. Conventionally, it may be an absorption systern wherein an adsorber oil, such as kerosene, is used to absorb the hydrocarbons from the hydrogen. Alternatively, cryogenic means may be used which embody low temperature separation of the hydrocarbons from the hydrogen. In the practice of this invention it is distinctly preferred to use cryogenic means the operating conditions for which are well known and need not be discussed in this specification. The residual gas from the cryogenic units is the basic stock for the town gas. However, since this residual gas contains a major amount of hydrocarbons with respect to hydrogen, the caloriiic value is typically in excess of 900 B.t.u.s per standard cubic foot (B.t.u./s.c.f.).

Accordingly, this invention embodies the use of the carbon dioxide previously produced in the hydrogen plant to dilute the residual gas stream from the cryogenic unit suicient to produce a town gas having a predetermined caloritic value such as 500 or 900 B.t.u./s.c.f. These latter calorifie Values are the two most prevalent classes of town gas used by industry in the house-hold in the world today. However, this invention is not to be limited to specific calorific values since values above and below these limits may also be produced, if desired.

The town gas produced in accordance with this invention has a heating value, or calorific value, in the range of from 300 to 1000 B.t.u./s.c.f., and a specific gravity (air=l.00) in the range of 0.30 to 1.00. It is important that the specific gravity or calorific value be adapted to the orifices used in the household gas appliances as designed.

As used herein, the term town gas is defined to embody any combustible gas made and supplied for general fuel use and includes combustible gases having the specific physical properties, such as calorific value and specific gravity, discussed herein.

Additional features and advantages of the present invention will be apparent from the description which follows with reference to the appended drawing which provides an example illustrating one specific embodiment of the invention.

Referring to the drawing, a naphtha fraction boiling in the range of 300 F. to 400 F. is fed into the process via line 10 wherein it is split into a first fraction in line 12 which feeds the hydrogen plant, and a second fraction in line 11 which feeds the catalytic reformer.

vtemperature of the order of about 800 a normally gaseous stream comprising carbon dioxide in line 17. It is to be understood that the hydrogen plant 14 contains sufficient equipment including purification and separation means, not shown, to effectuate the production of the hydrogen and carbon dioxide streams as specified.

Typically, in hydrogen plant 14 the gaseous mixture, of steam and vaporized naphtha is passed at a temperature from about l000 F. to 1500 F. over a conventional catalyst which has been maintained at a temperature in excess of 1200 F. The cracked gases leaving the reaction zone contain a major portion of hydrogen and carbon monoxide. These cracked gases are treated with excess steam in contact with an iron oxide catalyst at a F. to convert the carbon monoxide to carbon dioxide with the production of an equivalent amount of hydrogen. The carbon dioxide is separated from the hydrogen by conventional means such as absorption in triethanolamine.

The naphtha fraction in line 11 is passed via lines 13 and 18 into catalytic reformer 20 wherein the naphtha is reformed over a platinum catalyst at a temperature of about 900 F., a pressure of, say, 500 p.s.i.g., a weight hourly space velocity of, say, 2.0, and a hydrogen-tohydrocarbon mol ratio of, say, 4:1. Makeup hydrogen, as needed, may be added from the gaseous stream produced in hydrogen plant 14 via line 16 and may be added via hydrogen recycle from line 19.

The reformate leaving reactor 20 through line 21 is passed directly into dealkylator 22 without intervening separation. If necessary, additional heat may be added (or subtracted) from the reformate by external heater means or cooler means, not shown, depending upon the desired reaction temperature for the dealkylation reaction. As previously noted, the dealkylation reaction may be carried out either thermally or catalytically. For pur- Jposes of this example, it is distinctly preferred to use a catalyst consisting of approximately 10% to 15% by weight of chromium oxide on a high purity low sodium lcontent gamma-type alumina support.

The dealkylation temperature, for example, is about 1350 F. and an imposed pressure of approximately 500 psig. is maintained in the reactor. As noted, the reactor pressure may be Varied from about to 1000 p.s.i.g., but is dependent upon the pressure used for the catalytic reforming reaction. =It is distinctly preferred 'that the dealkylation reactor be maintained at a pressure of approximately equal to the pressure maintained for the reforming zone. If a higher dealkylation pressure is desired, conventional pumping means must be supplied in line 21, not shown. The hydrogen and vaporized reformate is contacted with the catalyst at a weight hourly space velocity of, say, 1.0 for a typical reaction time of `from 3 to 6 seconds, although neither of these figures is in any sense critical since time and temperature are interrelated to some extent.

Sufiicient hydrogen is already present in line 21 as part of the reformate to effect dealkylation. However, should additional hydrogen be needed or desired, it can be added via line 33. In any event, sufficient hydrogen must be present to saturate the alkyl groups removed from the alkyl benzenes, and to minimize the deposition of coke on 'the catalyst. It is to be noted that the dealkylation reaction to a considerable extent effectuates hydrocracking of the longer chain paraffin hydrocarbons present in the reformate stream.

Hydrogen 55.6 Methane 40.7 Ethane 3.7

As produced, the gas composition in line 26 has a gross calorific value of about 655. If desired, a portion of this gas in line 26 may be passed via lines 27, 31 and 32 into the town gas system. However, in accordance with this invention, a suiicient amount of carbon dioxide must vbe added to the gas in line 31 to reduce the caloriic value from 655 to 500. To effectuate this reduction, for every mol of gas in line 31 there must be added 0.31 mol of carbon dioxide to produce a town gas in line 32 having a caloritic value of 500.

Preferably, however, the gas in line 2e is lpassed, in total, to hydrogen purification unit 29 via line 28. In hydrogen purification unit 29 which operates under cryogenic conditions, the hydrocarbon portion is removed from the hydrogen by low temperature separation such that a hydrogen recycle stream comprising at least 80% by volume, preferably 90% `by volume, hydrogen, e.g. 97.5% hydrogen, in line 19. The residual gas stream from hydrogen purification unit 29 via line 30 has the following approximate composition:

` Mol percent Hydrogen 10.0 Methane 81.8 Ethane 8.2

The gas composition in line 30 has an approximate gross caloric value of 1003 B.t.u. s.c.f. The gaseous stream in line 30 is passed into the town gas system via lines 31 and 32. However, since the caloric value of this gas exceeds the predetermined set caloric value of 500, sufcient carbon dioxide is added to the material in line 31 via line 17 to produce a town gas in line 32 having a calorific value of 500. To effectuate the proper reduction in caloric value for each mol of gas passing from lines 30 and 31 into line 32, there must be added one (1) mol of carbon dioxide from line 17.

The invention claimed is:

1. Process which comprises converting a hydrocarbon fraction under conditions sufficient to produce a hydrogenrich stream and a carbon dioxide-rich stream, reforming a hydrocarbon stream containing naphthenic and paraffinic hydrocarbons under conditions including the presence of at least a portion of said hydrogen-rich stream suiiicient to produce a reformate containing alkylaromatic hydrocarbons, dealkylating the alkylaromatic hydrocarbons under conditions sufficient to produce (i) an eiiiuent containing aromatic hydrocarbons, and,

(ii) a normally gaseous stream of relatively low purity hydrogen gas;

separating at least a lportion of said hydrogen gas into a relatively high purity hydrogen stream and a light paraffinic hydrocarbon stream, recycling said purified hydrogen stream to the reforming step as part of said conditions, admixing at least a yportion of the carbon dioxide-rich stream with said light hydrocarbon stream in an amount sufficient to produce a town gas having a predetermined caloric value, recovering aromatic hydrocarbons in high concentration from said effluent, and recovering the town gas so produced.

2. Process according to claim 1 wherein the entire effluent from the reforming step is rpassed directly to the dealkylation step without intervening separation.

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3. Process according to claim 2 wherein said dealkylation conditions include the presence of a catalytic mass.

4. Process according to claim 1 wherein said hydrocarbon fraction and said hydrocarbon stream are naphtha fractions.

5. Process according to claim 2 wherein said hydrocarbon fraction to be converted and said hydrocarbon stream to be reformed are derived from pretoleum, contain primarily naphthenic and paratllnic hydrocarbons, and boil within the range from 100 F. to 400 F.

5;. Process according to claim 5 wherein said low purity hydrogen gas is separated by cryogenic means and said predetermined caloric value is from 300 to 1000 B.t.u. s.c.f.

7. Combination method for producing town gas and benzene which comprises the steps of:

(a) steam-cracking a naphtha fraction to produce a normally gaseous stream comprising carbon dioxide and a gaseous stream comprising at least by volume hydrogen;

(b) catalytically reforming a naphtha fraction lboiling between F. and 450 F. and containing primarily naphthenic and parafnic hydrocarbons in the presence of at least a portion of said gaseous hydrogen stream under conditions including a ternperature from 500 F. to 1050 F., a `pressure from 50 p.s.i.g. to 1200 p.s.i.g., and a weight hourly space velocity from 0.2 to 40 sufcient to produce a reformate containing alkylbenzene hydrocarbons and normally gaseous parafnic hydrocarbons;

(c) subjecting said reformate to catalytic dealkylating conditions including the presence of hydrogen, a ternperature from 1000o F. to 1500 F., a pressure from 100 p.s.i.g. to 1000 psig., a weight hourly space velocity from 0.1 to 20.0, and a hydrogen-to-hydrocarbon mol ratio from 3:1 to 10:1, whereby alkylbenzene hydrocarbons are converted to benzene;

(d) separating the effluent from the hydrodealkylation step into a liquid hydrocarbon stream comprising benzene and a gaseous fraction comprising from 40% to 70% by volume hydrogen;

(e) separating at least a portion of said gaseous fraction by cryogenic means into a hydrogen recycle stream comprising at least 80% by volume hydrogen and a light parafnic hydrocarbon stream;

(f) `passing said recycle stream to the catalytic reforming step (step b) as part of said conditions; and

(g) admixing light parafnic hydrocarbons from step (e) with at least a portion of the carbon dioxide stream from step (a) to produce a town gas having a predetermined caloric value.

`8. Method according to claim 7 wherein said predetermined calorific value is from 300 to 1000 B.t.u./s.c.f.

9. Method according to claim 8 wherein said light parainic hydrocarbons comprise methane and ethane.

References Cited UNITED STATES PATENTS 6/1966 Drehman et al. 260-668 DELBERT E. GANTZ, Primary Examiner.

G. E. SCHMTTKONS, Assistant Examiner. 

